1. Field of the Invention
This invention relates to a camera that is capable of long-exposure photography, and particularly to a noise reduction process that reduces noise caused by long-exposures.
2. Description of the Related Art
A digital camera configured for long-exposure photography can be used for shooting night scenes, fireworks, celestial bodies, and so on. This so called “bulb shooting” photography allows a trace of a star's movement or the headlights of the vehicle to be imaged on a picture.
In bulb shooting, dark current occurs as noise in an image sensor such as a CCD. The amount of dark current in bulb shooting is greater than that from normal shooting. Also, the value of dark current is different in each pixel. To reduce this fixed pattern noise, an image sensor is exposed in a state in which light is completely blocked after a long exposure is carried out (hereinafter, the first exposure will be referred to as a “main exposure” and the second exposure as a “dark exposure”). A noise component included in image signals is sampled from the difference between output signals of the main exposure and output signals of the dark exposure, so that the noise component can be removed. This process is described, for example, in JP2000-209506A.
Moreover, the temperature of an image sensor increases as the length of exposure increases. Accordingly, dark current increases with increasing period of exposure. One method of reducing a noise component that increases with temperature is to repeatedly take a dark exposure. Then, a noise component can be calculated from the average of a series of detected dark currents. This process is described in U.S. Pat. No. 7,636,113.
When the ambient temperature is extremely low at a photography location, the increase in the temperature of the image sensor is restricted. For example, when photographing a celestial body using bulb shooting at a high-altitude location with a low ambient temperature, the temperature of the image sensor decreases as the exposure time passes. The dark current output level depends upon the temperature of the image sensor; therefore, the lower the temperature, the greater the decrease in the output level of the dark current. Consequently, a dark exposure should last long enough to obtain dark current components accurately, and a totally long photograph time losses convenience.